1. Field of the Invention
The present invention relates to processes for coating metals, and particularly to laser gas assisted nitriding of alumina surfaces.
2. Description of the Related Art
Nitriding is the process of adding nitrogen to the surface of metals for improved hardness and wear resistance. A nitride coating may be applied to many different metals, including steel and other ferrous metals, titanium, molybdenum, and aluminum. Several processes are known for applying the nitride coating, such as gas nitriding, liquid or salt bath nitriding, and ion or plasma nitriding. Conventional gas nitriding generally uses ammonia, which dissociates to form nitrogen and hydrogen when brought into contact with a heated metal workpiece. Other processes are generally performed at high temperatures and/or pressures, and require substantial periods of time to form the nitride surface to the desired depth.
Laser gas assisted nitriding is a relatively new process for applying a nitride coating to metals. The process generally involves exposing the workpiece to laser radiation for comparatively brief periods in the presence of an inert gas. Nitrogen is supplied under pressure. Laser nitriding has several advantages, including the ability to apply a very thin coating; the ability to apply the coating in a vary narrow beam, if desired; relatively low temperatures and pressures to avoid deformation of the metal; and quick processing times, with exposure to radiation often being less than a second.
Alumina (Al2O3) finds wide application in industry due to its tribological and thermal properties, such as corrosion resistance and thermal stability at high temperatures. Alumina is widely used in lined process piping, chutes, cyclones, lined metal fabrication, and grinding mill components. This is due to the fact that all of these applications require a unique combination of hardness, extremely high abrasion resistance, and high strength over a broad range of temperatures. Moreover, alumina tiles are formed from fine alumina powders through sintering. The tiles have high porosity, particularly in the surface region. Alumina tiles are, in general, produced from microsized alumina powders. Depending on the powder size, in some cases, structural non-homogeneity and abnormalities, such as scattered small voids, are formed in the tiles. These abnormalities in the structure, particularly in the surface region, can be minimized through controlled laser melting.
However, during the laser melting process, an assisting gas is used to prevent excessive oxidation reactions taking place in the irradiated region. The oxidation reactions are exothermic, increasing the surface roughness and crack formation through subsequent high heating and cooling rates in the irradiated region. The use of the inert gas, such as nitrogen, in the laser melting process prevents surface defects that would otherwise result from high temperature oxidation reactions. Moreover, nitrogen at high temperatures and pressures forms AlN in the surface region, thus modifying the surface hardness. Because alumina and alumina-based composites have oxygen incorporated therein, it would be desirable to have a process that produces a hard, wear resistant nitride surface that is also not prone to developing cracks.
Thus, laser gas assisted nitriding of alumina surfaces solving the aforementioned problems is desired.